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Q: I have a bad case of string algae in my water garden and someone told me it was because I had too many fish. I have an approximately 1,000 gallon water garden and around 40 koi 6″ long in it. Is that really too many? If so, how many should I have? – Marco of Texas

A: You can ask anyone here at The Pond Guy; usually the first question we ask when someone says they have a bad algae problem is, “How many fish do you have?”, followed by, “What size water garden do you have?”. 9 out 10 times, there are way too many fish in the water garden. So why does having that many fish cause algae? Let me explain.

Sunlight + Food Source = Algae: Algae really only needs two things to grow, sunlight and a food source. The food source can come from many sources but fish waste is a major contributor. This means the more fish you have, the more waste, the more algae. Make sense?

Finding a Balance: When you put fish into your water garden always consider the future. Small fish become big fish and fish are very romantic creatures. Let’s say you purchased 20 koi at around 3″ each. Your pond may be able
to handle the fish load for a few seasons then all things start to change. As time went by, your fish have grown and maybe even started a family. As nature takes its course, your pond starts to pay the price and water quality becomes an issue. Take into consideration that 40 1″ fish produce the waste of just one 12″ fish. So even though you pond may have been able to handle the fish load in the past, you must consider that your fish load or fish waste grows expotentially every year.

Koi vs. Goldfish: I know that when you’re shopping for fish koi are more expensive and sometimes the goldfish look really nice too. I feel the same way myself at times. Just keep in mind that goldfish can reproduce up to 6 times a year where koi only reproduce once a year.

Fish Tips: Goldfish can grow up to 18” long and live up to 20 years, where as a koi can grow up to 36” long and live over 200 years! One famous scarlet koi, named “Hanako” (c. 1751 – July 7, 1977) was owned by several individuals, the last of which was Dr. Komei Koshihara. Hanako was reportedly 226 years old upon her death. Her age was determined by removing one of her scales and examining it extensively in 1966. She is (to date) the longest-lived koi fish ever recorded (wikipedia).